Filling level measuring device operating with microwaves, having an insert composed of a dielectric, and process for producing the dielectric

ABSTRACT

A description is given of a filling level measuring device operating with microwaves, having a housing and an insert composed of a dielectric, and of a process for producing the dielectric, in which the dielectric constant of the insert is adjustable and in which the insert has a high chemical resistance and a mechanical strength adequate for industrial applications. The dielectric is a composite material composed of a fluoroplastic, in particular polytetrafluoroethylene, and ceramic and is produced by mixing powdered ceramic and powdered fluoroplastic, drying the mixture, pressing the mixture and sintering the pressed mixture.

FIELD OF THE INVENTION

The invention relates to a filling level measuring device operating withmicrowaves, having a metallic housing portion through which microwavesare transmitted and/or received and in which an insert composed of adielectric is arranged. Furthermore, the invention relates to a processfor producing the dielectric.

BACKGROUND OF THE INVENTION

In filling level measurement, microwaves are sent by means of an antennato the surface of a filled substance and the echo waves reflected at thesurface. are received. An echo function representing the echo amplitudesas a function of the distance is formed and used to determine theprobable useful echo and its delay time. The delay time is used todetermine the distance between the surface of the filled substance andthe antenna.

All known methods which make it possible to measure relatively shortdistances by means of reflected microwaves can be used. The mostwell-known examples are pulsed radar and frequency-modulationcontinuous-wave radar (FMCW radar).

In the case of pulsed radar, short microwave transmission pulsesreferred to in the following as wave packets, are transmittedperiodically, reflected by the surface of the filled substance andreceived again after a distance-dependent delay time. The receivedsignal amplitude as a function of time represents the echo function.Each value of this echo function corresponds to the amplitude of an echoreflected at a particular distance from the antenna.

In the case of the FMCW method, a continuous microwave which isperiodically frequency-modulated linearly, for example on the basis of asawtooth function, is transmitted. The frequency of the received echosignal therefore has with respect to the instantaneous frequency whichthe transmitted signal has at the instant of reception a frequencydifference which depends on the delay time of the echo signal. Thefrequency difference between transmitted signal and received signal,which can be obtained by mixing the two signals and evaluation of theFourier spectrum of the mixed signal, consequently corresponds to thedistance of the reflecting surface from the antenna. Furthermore, theamplitudes of the spectral lines of the frequency spectrum obtained byFourier transformation correspond to the echo amplitudes. Therefore, inthis case, this Fourier spectrum represents the echo function.

Filling level measuring devices operating with microwaves are used invery many branches of industry, for example in chemistry or in the foodindustry. Typically, the filling level in a container is to be measured.These containers generally have an opening, at which a connection pieceor a flange is provided for the fastening of measuring devices.

In industrial measuring technology, dielectric rod antennas and hornantennas are regularly used for transmitting and/or receiving.Typically, a pot-like housing which has the geometry of ashort-circuited waveguide is used. An exciter pin, via which microwavesare transmitted and/or received through the housing, is inserted intosaid housing. In the case of a horn antenna, the housing is adjoined bya funnel-shaped portion which opens out in the direction facing thecontainer and forms the horn. In the case of the rod antenna, a rodcomposed of a dielectric and pointing into the container is provided.The interior space of the housing is usually filled virtually completelyby an insert composed of a dielectric. In the case of the horn antenna,the insert has a conical end, pointing into the container. In the caseof rod antennas, the insert is adjoined by the rod-shaped antenna.

On account of the dimensioning of the waveguide and the dielectricconstant of the insert, only certain modes can be propagated. Forfilling level measurements, modes having a radiation characteristic witha pronounced forward lobe are preferred, in the case of circularwaveguides, for example, the transverse electric (TE-11) mode. Thetransmission frequency is also prescribed in most countries.

In order that the dimensions of the housing are nevertheless variablewithin certain limits, for example to perform adaptations to dimensionsof containers, a dielectric with a substantially continuously adjustabledielectric constant is advantageous. In the following text, dielectricconstant always refers to the dielectric constant which is based on thevacuum dielectric constant and the value of which is equal to thequotient of the dielectric constant divided by the vacuum dielectricconstant.

In DE-A 44 05 855 there is described a filling level measuring deviceoperating with microwaves

having a metallic housing portion,

through which microwaves are transmitted and/or received and

in which an insert composed of a dielectric is arranged.

It has a rod antenna and the insert and rod antenna are composed of adielectric. It is specified to use plastic, glass or ceramic or amixture of said materials for this purpose.

Insert can come into contact with a medium located in the container.Depending on the application, this may well be an aggressive medium.Consequently, the insert should have in addition to the mechanicalresistance required for industrial applications also a high chemicalresistance.

In the case of commercially available filling level measuring devicesoperating with microwaves, polytetrafluoroethylene (PTFE), which has ahigh chemical resistance, is often used for this reason. The dielectricconstant of polytetrafluoroethylene (PTFE) is not variable, however.

In U.S. Pat. No. 5,227,749 microwave circuits and components aredescribed, for example microwave striplines, in which desired electricaland mechanical properties are achieved simultaneously by using anenclosure filled with a dielectric. The enclosure offers adequatemechanical stability, so that the dielectric can be selected purely onthe basis of its dielectric properties.

Although such a construction represents a feasible approach in the caseof microwave striplines and microwave circuits, it is unsuitable howeverfor use in an antenna. The housing and insert act as a waveguide inwhich the microwaves form. An enclosure would have different dielectricproperties than the dielectric embedded in it and, owing to itsnonisotropic electrical properties, would consequently lead toconsiderable disturbances in the desired modes during transmittingand/or receiving.

U.S. Pat. No. 4,335,180 there is described a dielectric for microwavecircuit boards and a method of making it.

The dielectric consists of polytetrafluoroethylene (PTFE), a fillermaterial and a fibrous material. The proportion of filler material isspecified as 10 to 75 percent by weight. Among the materials specifiedas, the filler material is aluminum oxide. The proportion of fibers isbetween 2.5 and 7 percent by weight of the dielectric and ensures itsmechanical stability. The dielectric constant of the material isspecified as 10 to 11.

This dielectric is made by blending the filler material and fibrousmaterial into a polymer dispersion. A flocculant is added to the slurrythus formed until a dough-like material is produced, which is thenshaped and dried.

In a circuit board, the fibers can be aligned in a plane by appropriateprocessing, for example pressing or rolling, so that a substantiallyhomogeneous thin sheet, that is a substantially two-dimensionalformation, is produced. A three-dimensional body cannot be readilyproduced in this way, however. In a three-dimensional body, fiberscannot be aligned in one plane by pressing or rolling. Raised fibersbehave like small quills and the body remains correspondingly porous andinhomogeneous in spite of pressing. It would consequently have lessmechanical strength and inhomogeneities would lead to reflections of themicrowaves. There is also the risk of the porous material beingsaturated with moisture. Moisture in the material leads to a high lossfactor tan δ.

SUMMARY OF THE INVENTION

It is an object of the invention to specify a filling level measuringdevice operating with microwaves, having a housing and an insertcomposed of a dielectric, and a process for producing the dielectric, inwhich the dielectric constant of the insert is adjustable and in whichthe insert has a high chemical resistance and a mechanical strengthadequate for industrial applications.

For this purpose, the invention comprises a filling level measuringdevice operating with microwaves

having a metallic housing portion,

through which microwaves are transmitted and/or received and

in which there is arranged an insert composed of a dielectric, whichconsists of a composite material composed of a fluoroplastic, inparticular polytetrafluoroethylene (PTFE), and ceramic.

According to an advantageous refinement, the composite material has aproportion of ceramic which is below the percolation limit.

According to a further refinement, the composite material has adielectric constant ε and the quotient of the dielectric constant ε andthe vacuum dielectric constant ε₀ has a value between 2 and 10.Furthermore, the composite material preferably has a loss factor tan δwhich is less than one fiftieth.

According to an advantageous development, the insert has in a portion ofthe housing arranged in the direction of transmission a lower proportionof ceramic than in a portion facing away from the direction oftransmission.

Furthermore, the invention includes a process for producing a compositematerial from fluoroplastic, in particular polytetrafluoroethylene(PTFE), and ceramic, which comprises the following steps:

a) producing a mixture of powdered ceramic and powdered fluoroplastic,

b) drying the mixture,

c) pressing the mixture and

d) sintering the pressed mixture.

According to an advantageous refinement of the process, the proportionof ceramic in the mixture is below the percolation limit.

According to a further refinement of the process, the quotient of thedielectric constant ε of the composite material and the vacuumdielectric constant ε₀ has a value between 2 and 10 and the compositematerial has a loss factor tan δ which is less than one fiftieth.

According to a further refinement of the process, two or more mixtureswith different proportions of ceramic are produced, and the mixtures arelayered one on top of the other before pressing in such a way that theproportion of ceramic in the composite material decreases from layer tolayer.

The invention and further advantages are now explained in more detailwith reference to the figures of the drawing, in which two exemplaryembodiments of a filling level measuring device operating withmicrowaves are represented; identical parts are provided in the figureswith identical reference numbers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a longitudinal section through a first exemplary embodimentof a filling level measuring device operating with microwaves; and

FIG. 2 shows a longitudinal section through a second exemplaryembodiment of a filling level measuring device operating withmicrowaves.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

In FIGS. 1 and 2, a longitudinal section through a filling levelmeasuring device 1 operating with microwaves and to be fastened on acontainer is diagrammatically represented in each case. In thecontainer, not represented in the figures, there is a medium and thefilling level measuring device 1 serves the purpose of determining thefilling level of this medium in the container. To this end, in the caseof the exemplary embodiment of FIG. 1, microwaves are transmitted intothe container via a rod-shaped antenna 2 a, pointing into the container,and the echo waves reflected at the surface of the filled substance arereceived.

In the case of the exemplary embodiment represented in FIG. 2, a hornantenna is provided. The latter has a funnel-shaped horn 2 b made of ametal, in particular a high-grade steel, which opens out in thedirection facing the container.

In both exemplary embodiments, the measuring device has in each case acylindrical housing 1. In the case of the exemplary embodiment of FIG.1, the housing 1 is provided with an external thread 11, by means ofwhich it is screwed into a flange 3. The latter is mounted on thecontainer on a corresponding counter-flange 4. In the case of theexemplary embodiment of FIG. 2, the housing 1 is likewise screwed intothe flange 3. The horn 2 b is screwed onto the flange 3 at a later time.

The housing 1 has the shape of a pot or of a tube closed off on one sideat the end. The microwaves are generated by a microwave generator (notrepresented) and are fed via a coaxial line 5 to an exciter element 6,introduced laterally into the housing 1. It goes without saying that itis also possible to introduce the exciter element into the housing fromone of the end faces. The microwave generator is, for example, apulsed-radar device, an FMCW device or a continuously oscillatingmicrowave oscillator.

The housing 1 consists of an electrically conductive material, forexample aluminum or high-grade steel. The microwaves are transmittedand/or received through the housing 1 via the antenna 2 a or 2 b.

In the case of both exemplary embodiments, in the housing 1 there isarranged an end element 7, which completely fills an interior space ofthe housing 1 facing away from the container, apart from a recess whichserves for receiving the exciter element 6. On the side facing thecontainer, a cone is formed onto the end element 7. An interior space ofthe housing 1 adjoining said cone is filled by a substantiallycylindrical insert 8. The insert has on its side facing the end elementa recess which is identical in shaped to the cone. The insert 8 isscrewed into the housing 1 by means of a thread 81.

In the direction facing the container, there is formed onto the insert 8a portion 82 of smaller external diameter. This portion has an externalthread 83. In the case of the exemplary embodiment of FIG. 1, therod-shaped antenna 2 a is screwed onto this external thread 83. For thispurpose, the antenna 2 a has a correspondingly shaped recess, providedwith an internal thread. In the case of the exemplary embodimentrepresented in FIG. 2, a conical end piece 9, pointing in the directionof the container, is screwed onto the portion 82.

The sealing of the container takes place in the case of the exemplaryembodiment of FIG. 1 by means of an annular disk 2 al which extendsradially outward, is formed onto the rod-shaped antenna 2 a and isclamped between the flange 3 and the counter-flange 4. In the case ofthe exemplary embodiment of FIG. 2, an annular disk-shaped seal 10 isprovided, which is likewise clamped between the flange 3 and thecounter-flange 4.

The insert 8 consists of a dielectric, which is a composite materialcomposed of a fluoroplastic and ceramic. A fluoroplastic is understoodto mean a fluorine-containing polymer, i.e. a polymer with a highproportion of fluorine. The fluoroplastic is preferablypolytetrafluoroethylene (PTFE). Likewise very well-suited aremodifications of polytetrafluoroethylene (PTFE) in whichpolytetrafluoroethylene (PTFE) serves as the basic substance.

Examples of this are tetrafluoroethylenehexafluoropropylene copolymer(FEP) and perfluoroalkoxy copolymer (PFA). The following descriptiontakes polytetrafluoroethylene (PTFE) as an example. This is not to beregarded as a restriction, however.

The end piece 7 likewise preferably consists of this material. In thecase of a horn antenna as is represented in FIG. 2, the conical endpiece 9 also preferably consists of this composite material.

The proportion of ceramic is preferably below the percolation limit.Below the percolation limit there is no continuous link between theparticles of ceramic in the three spatial directions. Depending on theparticle size, proportions of ceramic of up to 35 percent by volume arepossible as a result.

This achieves the effect that the particles of ceramic are firmlyembedded in the polytetrafluoroethylene (PTFE). The composite materialconsequently has a mechanical strength which substantially correspondsto the strength of polytetrafluoroethylene (PTFE). The material ishomogeneous and has a low porosity.

The percolation limit depends oh the size of the particles of the twocomponents and can be determined experimentally by determining thedielectric constant or the resistivity of the material as a function ofthe proportion of ceramic. At the percolation limit, a distinctnonlinear increase in these parameters can be noted.

The ceramic is preferably an aluminum oxide (Al₂O₃), for examplecorundum. However, barium titanate (BaTi₄O₉), calcium titanate (CaTiO₃)or aluminosilicates can also be used.

The dielectric constant of aluminum oxide (Al₂O₃) has a value ofapproximately ε/ε₀≅7; in the case of barium titanate (BaTi₄O₉), thisvalue is ε/ε₀≅50 and in the case of calcium titanate (CaTiO₃) a value ofε/ε₀≅40 to 60 can be obtained. Polytetrafluoroethylene (PTFE) has adielectric constant of ε/ε₀≅2.

The percolation limit for composite material composed ofpolytetrafluoroethylene and ceramic is at a proportion by volume ofabout 33% ceramic if the size of the particles of the two components,ceramic and polytetrafluoroethylene (PTFE), is approximately the same.

The dielectric constant of the composite material can be determined toan approximation by linear interpolation. The dielectric constant of thecomposite material is consequently approximately equal to the weightedsum of the dielectric constants of polytetrafluoroethylene (PTFE) andceramic, the weighting factors being equal to the proportions by volumeV in percent by volume of the components

ε/ε (composite material)≅ε/ε(ceramic)*V(ceramic)+ε/ε(PTFE)*V(PTFE)

The actual values of the dielectric constant ε/ε₀ of the compositematerial with a proportion of ceramic below the percolation limit areslightly below the values determined by linear interpolation.

If aluminum oxide (Al₂O₃) is used, dielectric constants with values ofε/ε₀≅2 to ε/ε₀≅5 can be adjusted; if barium titanate (BaTi₄O₉) is used,the adjustable values lie between ε/ε₀≅2 and ε/ε₀≅33 and, if calciumtitanate (CaTiO₃) is used, they lie between ε/ε₀≅2 and ε/ε₀≅30.

Preferably, the value for the dielectric constant ε/ε₀ lies between 2and 10. As a result of the low dielectric constant, housings 1 with arelatively large internal diameter can be used.

In the case of a dielectric constant of ε/ε₀≅4, an internal diameter ofabout 2 centimeters can be used for transmitting and/or receivingmicrowaves at a frequency of about 6 GHz. This offers the advantage thatinevitable production-related tolerances of the components have minoreffects.

A further great advantage of the composite material is that, although ithas approximately the mechanical strength of polytetrafluoroethylene(PTFE), the composite material nevertheless has a very much lowercoefficient of thermal expansion than polytetrafluoroethylene (PTFE).

The coefficient of thermal expansion of polytetrafluoroethylene is about150*10⁻⁶. The housings 1 typically consist of a high-grade steel.High-grade steel has a coefficient of thermal expansion of 17*10⁻⁶. Thecoefficient of thermal expansion of ceramic is of the same order ofmagnitude as the coefficient of thermal expansion of metal. Thecoefficient of thermal expansion of a composite material consequentlywill be much less than the coefficient of thermal expansion ofpolytetrafluoroethylene (PTFE), according to its proportion of ceramic.

It is ensured by the proportion of ceramic that the insert 8 and housing1 experience a comparable thermal expansion, so that very much lowertemperature-dependent mechanical stresses occur in the housing 1. Thecomposite material also has a lower pressure- and temperature-dependenttendency to flow than is the case with polytetrafluoroethylene (PTFE).The measuring device can be correspondingly used at a higher thetemperatures and pressures. In comparison with the use of pure ceramic,the composite material additionally offers the advantage that, onaccount of the polytetrafluoroethylene (PTFE), it is not brittle. Thereis consequently the possibility of also using relatively largecomponents, such as the rod-shaped antenna 2 a, composed of thismaterial. The use of a hard brittle antenna, for example of pureceramic, would be problematical, since the antenna could break off undermechanical loading.

The composite material has a loss factor tan δ which is less than onefiftieth. It is ensured by the low loss factor that the microwave powerloss is low.

In the case of a rod-shaped antenna 2 a, as is represented in FIG. 1, itmay be desired that the rod-shaped antenna 2 a consists ofpolytetrafluoroethylene (PTFE). This is the case, for example, whenevera measuring device is to be equipped at a later time with an insert 8composed of the composite material, for example because the compositematerial has a more favorable dielectric constant or because thecustomer would like to use polytetrafluoroethylene (PTFE) exclusively inthe container on account of the chemical properties of its filledsubstance.

To avoid mechanical stresses on account of the different coefficients ofthermal expansion of the materials and deformations caused as a result,the insert 8 preferably has in a portion of the housing 1 arranged inthe direction of transmission, here in the direction facing away fromthe antenna, a higher proportion of ceramic than in a portion arrangedin the direction of transmission, here facing the antenna. There isconsequently a virtually continuous, transition, by means of which theadvantages of the composite material can be utilized without suddenchanges in impedance occurring, which would lead to reflections ofmicrowaves and/or a greater loss factor tan δ.

A composite material composed of ceramic and fluoroplastic, preferablypolytetrafluoroethylene (PTFE), is produced by annealing the powderedceramic, for example aluminum oxide, for example corundum, or some otherceramic, at 800° C. This ensures the detachment of any attached hydroxylgroups.

Here too, the description takes polytetrafluoroethylene as an example.This is not to be regarded as a restriction to this material. Thestatements made above with respect to fluoroplastics applycorrespondingly.

In a next step, polytetrafluoroethylene powder and powdered ceramic aremixed at room temperature. The next process step comprises drying themixture at 100° C. to 150° C. and pressing the dried mixture into thedesired shape under pressure of 500 kg/cm² to 1000 kg/cm² at roomtemperature. The pressed blank is sintered for at least five to sixhours at 375° C. to 400° C.

If Al₂O₃ is used, the powdered material should initially be annealed atabout 1250° C., subsequently ground for 12 hours, with the addition ofwater, at room temperature and then dried for 12 hours at 100° C. to150° C., before the procedure specified above is commenced.

A composite material in which the proportion of ceramic has a gradientcan be produced by means of the described process by producing two ormore mixtures with different proportions of ceramic, and layering themixtures one on top of the other before pressing in such a way that theproportion of ceramic in the composite material decreases from layer tolayer.

What is claimed is:
 1. A filling level measuring device operating withmicrowaves, having a metallic housing (1), through which microwaves aretransmitted and/or received and in which there is arranged an insertcomposed of a dielectric, which consists of a composite materialcomposed of a fluoroplastic and ceramic.
 2. The filling level measuringdevice operating with microwaves as claimed in claim 1, in which thecomposite material has a proportion of ceramic which is below thepercolation limit.
 3. The filling level measuring device operating withmicrowaves as claimed in claim 1, in which the composite material has adielectric constant ε and the quotient of the dielectric constant ε andthe vacuum dielectric constant co has a value between 2 and
 10. 4. Thefilling level measuring device operating with microwaves as claimed inclaim 1, in which the composite material has a loss factor tan δ whichis less than one fiftieth.
 5. The filling level measuring deviceoperating with microwaves as claimed in claim 1, in which the insert (8)has a first portion oriented in the housing (1) in the direction oftransmission and a second portion facing away from the direction oftransmission, the first portion having a lower proportion of ceramicthan the second portion.
 6. The filling level measuring device operatingwith microwaves as claimed in claim 1, in which the fluoroplastic ispolytetrafluoroethylene (PTFE).